Comets, like the planets and moons, were born from the cloud of dust and gas before the Sun flared into existence. Often described as “dirty snowballs”, they consist of a solid nucleus up to ten kilometres across, with a vaporous tail that can be millions of kilometres in length. The dramatic tail is actually composed of two, often overlapping parts: a curved dust tail trailing behind, and a gas tail pushed by the solar wind pointing directly away from the Sun.

The observed comets are classified as either short or long period. The short period comets orbit in regions of space between 35 and 100 astronomical units (AU) from the Sun; each AU is the distance from Earth to the Sun, or about 150 million kilometres. Just beyond the orbit of Neptune, these regions known as the Kuiper Belt and the Scattered Disk are filled with objects similar to the asteroids that exist between Mars and Jupiter. These comets are flung onto their dramatic orbits by the gravity of Neptune, and every so often one is deflected towards the Earth and the Sun.

Even more exotic are the long period comets from the Oort Cloud. This distant region of space between 3,000 and 50,000 AU – almost a quarter of the way to the closest other star – is hypothesised from the orbits of these far-travelling comets with orbital periods of millions of years. In it, trillion of comets are travelling on their distant ellipses, still gravitationally tied to the Sun. In some instances, such as Halley’s Comet, they are captured by the gravity of the outer planets and bound to much shorter orbits.

When a comet comes within the orbit of Mars, the energy of the Sun begins to heat up its atmosphere to form the dust and gas tails. When a comet comes close enough to Earth, we can train our telescopes on them. If we plan and cooperate, we can send robots after them to take a more careful look.

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Founded in 2011, The Public Domain Review is an online journal and not-for-profit project dedicated to promoting and celebrating the public domain in all its richness and variety. All works eventually fall out of copyright – from classics works of art to absentminded doodles – and in doing so they enter the public domain, a vast commons of material that everyone is free to enjoy, share and build upon without restriction. Our aim is to help our readers explore this rich terrain – like a small exhibition gallery at the entrance to an immense network of archives and storage rooms that lie beyond. With a focus on the surprising, the strange, and the beautiful, we hope to provide an ever-growing cabinet of curiosities for the digital age, a kind of hyperlinked Wunderkammer – an archive of materials which truly celebrates the breadth and variety of our shared cultural commons and the minds that have made it.